Differential signal lines can be used to achieve lower electromagnetic interference (EMI) emissions, better signal integrity (SI), and relatively better data quality for high-speed digital signals in high-speed network equipment. However, in addition to transporting the desired differential-mode signals, undesired common-mode noise can also propagate along differential signal lines, and conventional common-mode noise suppression filters—such as ferrite chokes, which work well in lower frequency ranges—have only limited effectiveness in the much higher gigahertz frequency ranges. Common-mode noise is particularly problematic at the specific gigahertz frequencies corresponding with the digital signal rates and harmonics of the system gigahertz-clock (specifically at 10.3125 GHz and 20.625 GHz).
Common-mode noise can originate in both input/output (I/O) differential buffers and printed circuit board (PCB) differential lines when, for example, differential signals are skewed in time and unbalanced in amplitude and rise-and-fall times. Furthermore, I/O ports and I/O cables may act as efficient slot and wire antennas, respectively, for common-mode noise. Consequently, it is important to suppress common-mode noise before it can reach I/O ports and I/O cables, but to do so without degrading the differential signals. Yet for implementations in densely populated PCBs with a high number of differential signal lines, the use of any single-differential filters may not be suitable since high-capacity 10-Gb/s network equipment may have hundreds of I/O differential signal lines that require common-mode suppression.
Some attempts have been made to achieve common-mode suppression in gigahertz frequency ranges using electromagnetic band-gap (EBG) techniques. In implementation, two general types of EBG structures are the “mushroom” and the “HIS.” Typical mushroom structures utilize periodic EBG cells (or “patches”) that are embedded into a multilayer PCB stackup between the power and ground reference planes to suppress common-mode noise; however, the size of EBG structures used in densely populated PCBs is crucially important, and traditional mushroom-like EBG structure are generally too large. In contrast, typical high impedance surface (HIS) structures comprise a combination of narrow slots and inductive stripes that are embedded in the power or ground reference planes, but slots in the power or ground reference planes can have the undesirable effect of degrading signal and/or power integrity.